What’s the difference between a one-alarm and a four-alarm fire?

What’s the difference between a one-alarm and a four-alarm fire?—Douglas Diarra, St. Lawrence Market

A one-alarm call is the basic response to a fire alarm, and usually includes two pumper trucks, a rescue unit, a ladder truck, and a chief to supervise things. If the first unit thinks it’s necessary, they’ll call in a second alarm, which will double the fire department’s response. Call a three-alarm fire and you’ll triple it. If it’s a three- or four-alarm fire, the department will also send out trucks that store extra oxygen and area lighting, media relations crews to wrangle journalists, and even a snack truck to keep firefighters fuelled. On average, the fire department deals with a two-alarm fire every day, a three-alarm fire every week, and a handful of four- or five-alarm fires a year—including the one that burned down Sassafraz in Yorkville last winter. Past that, the only other level is something called a general alarm, reserved for conflagrations on the scale of Toronto’s Great Fire of 1904, which razed more than 20 acres of the downtown. How many trucks does a general alarm involve? According to the fire department, “Everybody’s out, baby.”